Time division multiplex switching system for interconnecting telephone circuits which operate in accordance with different signalling systems and call formats

ABSTRACT

A time division multiplex switching system converts call formats and signalling protocols so that calls can be interchanged between circuits originating in different networks thereby providing a gateway for telephone transmissions originating from sources (e.g., foreign countries) having differences in transmission (signal attenuation characteristics and companding laws) and signalling and numbering plans or protocols. This switching system is characterized by a processor which is programmed to identify the circuits requiring interconnection and to process signalling messages in accordance with the signalling systems and formats used in the circuits, while translating the signalling messages into common, protocol independent signaling messages which are used for establishing time division multiplex connections in a time slot interchange which is connected to the circuits via PCM highways and for exchanging signalling information (e.g., as to seizure, detection and release) between the circuits; the signalling messages also containing information for conversion of PCM data messages which are switched in the time slot interchange thereby reconciling differences in transmission, such as the companding law used for PCM encoding and decoding and the attenuation (pad value) of the call data in the circuits. The circuits may, for example, be incoming and outgoing trunks.

The present invention relates to time division multiplex (TDM) switchingsystems for handling communications between circuits (port circuitswhich may be lines or trunks or service circuits for tones, ringing,etc.), and particularly to a TDM switching system which provides agateway between circuits which operate in accordance with differenttransmission and signalling formats and protocols.

The invention is especially suitable for making tandem or transitconnections between trunks which operate in accordance with different,disparate signalling systems and which may transmit calls with differenttransmission characteristics such as different PCM companding laws(μ-255 law or A-law) by performing companding law conversion andsignalling conversion, including numbering plan adaptation. It is afeature of this invention to provide such signalling, numbering plan andtransmission conversions within a time division switching system,thereby integrating the switching and conversion functions andpreventing interference between such functions and telephone exchange orswitching functions.

Different countries often use different signalling systems and formats.They also use disparate PCM encoding and decoding and insertion loss orpadding in order to translate calls into PCM data messages representingthe calls which are transmitted and received in different time slots sothat interconnections can be made by time division multiplexingtechniques. The signalling systems used in different countries mayinvolve the use of different numbers of digits and different numberingplans as well as other signalling messages to represent data or voicetransmissions.

Heretofore, special interfaces dedicated to trunks fromtelecommunications networks in foreign countries have been used totranslate the signalling messages and the data messages (PCM messagedata) so that they become compatible with the signalling systems andformats which are used in the telephone network of the receivingcountry. Differences in signalling systems have been an obstacle,particularly in small nations, such as the Pacific Ocean Islands toreceiving international telephone communications.

It is the principal object of the present invention to provide animproved time division switching system for PCM communications whichintegrates the switching function with the conversion functionsnecessary to reconcile the differences in transmission, signalling(including number plan), protocols and formats used in differenttelephone networks thereby providing, at an affordable cost, thefacility for small telephone systems such as located on remote islandsand in developing countries to be connected to receive internationaltelecommunications service.

It is a more specific object of the present invention to provide animproved time division switching system for PCM data messages which areto be connected to trunks, lines or other circuits operating inaccordance with different switching protocols and to provideinterconnections therebetween without deterioration or interference withcommunications between such networks due to the interposition ofsignalling conversions, companding law conversions and other conversionsnecessary to reconcile the differences between formats and protocolsused in such different circuits.

It is a still further object of the present invention, to provide animproved time division multiplex switching system in which circuits(ports, trunks, lines, etc.) may be interconnected without the need fordifferent signalling messages and formats to route calls and to exchangesignalling information between the circuits, such routing and signallingdata (call treatment data) being translated into a universal, protocolindependent language.

Briefly described, a time division switching system embodying theinvention provides communications between circuits which operate withsignalling data signals formatted according to different signallingprotocols and which are connected to PCM highways over which messagedata is transmitted. The system uses a time slot interchange connectedto the highways which provides time division multiplex connections forthe message data. First control means responsive to signalling datasignals from the circuits controls the circuits in accordance with thesignalling protocol in which the signalling data is handled by thesecircuits. Second control means translates the signalling data intosignalling messages in accordance with the protocol common to thecircuits for establishing the time division multiplex connections in thetime slot interchange and for transmission of signalling messagesbetween the first control means for use in operating the circuits inaccordance with their individual, unique signalling protocols. In thisway circuits in different telephone networks using different numbers ofdigits (or other numbering plans), decadic (dial pulse) ormultifrequency (DTMF) signalling protocols (R1, R1 modified, R2 SS5,SS7, CEPT and other signalling protocols) may be interconnected througha common time slot interchange with signalling messages establishing theinterconnections in a common language, thereby in a simple and effectivemanner reconciling differences in the signalling systems and formats andalso providing for conversion of the PCM codes in accordance with thecompanding laws (μ-255 law or A law) or insertion loss (padding) used inthe telephone networks, the circuits of which are interconnected.

The foregoing and other objects, features and advantages of theinvention as well as a presently preferred embodiment thereof willbecome more apparent from a reading of the following description inconnection with the accompanying drawings in which:

FIG. 1 is a simplified block diagram of a time division multiplexswitching system for PCM data messages which embodies the invention;

FIG. 2 is a functional block diagram illustrating the operation of thesystem shown in FIG. 1;

FIG. 3 is a more detailed functional block diagram illustrating thesystem shown in FIGS. 1 and 2;

FIG. 4 is a more detailed block diagram of the time slot interchangeshown in FIG. 1;

FIG. 5A is a portion of a state transition table and FIGS. 5B-Dconstitute a flow diagram illustrating the programing of the processorshown in FIGS. 1 and described in connection with FIGS. 2 and 3.

Referring to FIG. 1 there are shown circuits which are adapted to beconnected through PCM highways via a time slot interchange (TSI) 10.These circuits are shown by way of illustration as an incoming trunk 12and an outgoing trunk 14. The trunks may be of various types dependingupon their source, for example T1, CEPT, etc. The trunk circuits 12 and14 and the time slot interchange 10 are connected via a processor bus toa computer 16 consisting of a processor (a microcomputer chip) andmemory. The processor and memory constitute a state machine, variousstates of which are accessed in accordance with events taking place onthe trunks, such as seizures, releases, etc. The system enables a callto come in on one type of trunk, and which has transmissioncharacteristics (encoding into PCM in accordance with one kind ofcompanding law) and signalling in accordance with a particular protocol,to be connected via the time slot interchange (TSI) to a trunk ofanother type so as to provide two way communications therebetween.

The time slot interchange 10 has pad (insertion loss or attenuation)control and companding law conversion so as to transmit the PCM messagedata in accordance with the pad and companding law for the receiving orcalled trunk. Signalling is handled by translation of the signallingmessages, including their numbering plans, into a common, protocolindependent language (a universal language). This universal languagesets up the connection in the TSI 10, and also sets up the pad andcompanding law converter in the TSI. Thus upon detection of a call andidentification of the incoming trunk (or other circuit) and the outgoingtrunk (or other trunk), which may be members of trunk (or circuit)groups, signalling messages are generated which set up the routing andthe connection in the TSI. Upon release or the end of a call, a newuniversal message, based upon the events detected on the releasingtrunk, is transmitted to the TSI and the associated trunk to completethe call and release the connection.

The time slot interchange and the system used therein for establishingtime division multiplex connections (allocating time slots) is, in apresently preferred embodiment of the invention, similar to the TSIdescribed in U.S. Pat. No. 4,228,536 issued Oct. 14, 1980 to K.Gueldenpfennig and C.J. Breidenstein and assigned to RedcomLaboratories, Inc. It includes address registers which receive callrouting data. This call routing data is derived from the signallingmessages which are transmitted from the incoming trunk circuit. Thesignalling messages are translated by the processor 16 into universal,protocol-independent messages which address the TSI. These messages alsoinclude bits which control the pad and companding law conversion meansin the TSI 10. The incoming trunk may operate in accordance with μ-255law or A-law, while the outgoing trunk operates in accordance with thesame conversion law thus no conversion may be necessary (i.e., μ-255 lawto μ-255 law which is data transparent or A law to A law which is alsodata transparent). However, the companding laws may be different andμ-255 law companding used on the incoming trunk may need to be convertedto A law companding for decoding of the PCM data messages on theoutgoing trunk, or vice versa. The pad values and the companding lawconversion as required and as signalled by the universal signallingmessages are set up in the TSI 10 in order to accomplish the pad andcompanding law conversion functions.

The processor 16 operates to determine which circuit of the varioustypes of circuits which are connected to the TSI 10 has dialed in orotherwise requested service. Processor 16 also determines the type ofcircuit demanding service, thereby identifying the signalling protocol,including numbering plan used in the network containing that circuittype. The identification of the circuit also indicates the compandinglaw and pad value which is being used in that circuit type. Theprocessor invokes the states and calls up the routines necessary tooperate the circuits involved in the requested connection in accordancewith their unique protocols, while translating the signalling data intothe universal, protocol independent language to provide signalling andmessages carrying call set up data and other signalling data to the TSIand to the circuit to which the connections are to be made (the outgoingtrunk in the case illustrated in FIG. 1).

The circuit type may be identified by its hardware location or portidentification code (PID) as discussed in the above-referencedGueldenpfennig Breidenstein patent. This identification accessesinformation as to the type of circuit by its screen class. Then thetranslator is used to respond to dialed digits, thereby developing calltreatment data in the form of the universal signalling messages which goto the TSI and the outgoing trunk. At the outgoing trunk, the universalmessages are again translated into the protocol in which that circuittype operates. Accordingly, the switching function and the conversionsfunctions are integrated and do not interfere with each other, and theProcessing associated with a given type of circuit is not encumbered orcomplicated by its requiring knowledge of all of the various signallingprotocols of the other circuits to which it may be connected. Theprocessing for a given type of circuit need only be concerned with itsspecific protocol and the universal protocol by means of which itcommunicates with other circuit types.

The call treatment data defines how to handle the call. One treatmentmight be to route directly to a specific outgoing circuit. Anothertreatment may call up a process to wait for dialed digits (expecteddigits). Based on these digits, another signalling message in theuniversal language is generated and used to set up the TSI so as tointerconnect the outgoing circuit, which is dialed, to the incomingcircuit, which has requested the connection thereto. The translation isobtained through the use of a data base (tables) in memory of theprocessor 16 which are accessed as different events are detected.Different table data from the database is used for the protocol specificconnections to operate the circuits (the incoming and outgoing trunks)in accordance with their unique protocols. Other tables are accessed togenerate the call treatment data in the form of the universal signallingmessages which go to the TSI and which are transmitted between thecircuits (effectively) to evoke the unique protocol dependent operationstherein.

The processor, therefore, has first control means which are protocolspecific and operate the circuits in accordance with their own protocolsand second control means which effect the translations and generate thecall treatment data in the universal protocol independent language.Information in the call treatment data converts the numbering plans,thereby accommodating different digit prefix counts when outpulsing onthe selected circuit. In other words, the signalling messages invokeunique port event processor (PEP) routines specific to the port (e.g.trunk type). The tables in the processor memory constitute a matrix forall states in which the circuits can be placed in response to all eventsthat can occur. The call treatment identifies the events so thatcircuits having different screen classes can be operated in accordancewith a particular sequence using different subroutines, also stored inmemory, to carry out the port processes for each different type ofcircuit, thereby implementing the unique protocol for that circuit. Ineffect, the incoming trunk is transparent to the outgoing trunk. Bothtrunks receive the same call treatment data in response to the eventsand set up the corresponding states through which the PEP is executed.

In FIG. 2, a typical call set up in the system shown in FIG. 1 isillustrated. The incoming and outgoing trunks are of different type,namely trunk type "A" and trunk type "B". Each trunk type operates inaccordance with its own unique signalling protocol. An incoming seizureis detected. That seizure is processed by a state machine implemented inthe processor 16 which is unique to trunk type A. Further trunkprocessing can be carried out by the state machine as it executes itsdifferent states. The state machines constitute the first control meansreferred to above.

Another set of states executed by the processor translates thesignalling messages. This is shown as the protocol independent tandemtrunk call set up state machine for the illustrated case where a tandemor transit connection is being made between the type A and the type Btrunks. This call set up information is transmitted in the form of calltreatment data to the TSI 10 and also to the outgoing trunk,specifically to its unique state machine where the messages processedand any further processes are also carried out by the state machinewhich is unique to trunk type B. There is, however, transmitted, betweenthese unique state machines or processes, the signalling messages inaccordance with the universal (intermediate) language which is common toall trunk types.

Referring to FIG. 3 there is shown two sets of port circuits which areinterconnected through the time slot interchange 10 via PCM send andreceive highways. The port circuits 20 may be the incoming trunks andthe port circuits 22 connected to the receive highway may be theoutgoing trunks which are illustrated in FIG. 1. The signalling data(SIGDATA) includes port identification data and is applied to portidentification (ID) detectors 24 and 26 for the incoming port circuits20 and the outgoing port circuits 22 (respectively). The signalling dataproceeds to the port specific processor 28 where seizure and releaseevents are detected and processed. A similar port specific processor 30is associated with the outgoing port circuits 22. The port specificprocessors are in communication with translators 32 and 34. Thesetranslators carry out screen class detection based on the SIGDATA whichincludes codes identifying the port circuits requiring service or towhich connections are made. The translators generate the universalsignalling messages in the form of interprocessor messages (IPMS) whichare received by the translators and then operate their port specificprocessors 28 and 30. The call treatment data also sets up the routingbetween the selected circuits of the port circuits 20 and 22.

Referring to FIG. 4 there is shown the time slot interchange 10. Itincludes the apparatus which sets up the routing, namely the RAM whichreceives the incoming PCM on the send highway and connects it to thereceive highway so that the data message is read out in the time slotallocated to the outgoing port. This outgoing PCM, for example, 8 bitbytes, is applied to a post-processor. This post-processor is in theform of memory in the TSI 10 connected to the outgoing or receivehighway on which the switching data appears. This memory 42 contains apad and companding law conversion look up table. The conversion dependsupon the 8 bits of the PCM and the signalling message.

The signalling messages are applied to a pad and companding lawconversion control 44. This is a register which stores the bits of thesignalling message which control the selection of the pad and compandinglaw conversion, for example any of the four or more conversion discussedabove, namely A law to A law, μ-255 law to μ-255 law, μ-255 law to A lawand A law to μ-255 law. There may be, in addition, eight pad values. Theregister in the control 44 may also be the TSI address register andtherefore is shown connected to the basic TSI function random accessmemory (RAM) in which the incoming PCM is placed. For four differentpossible companding law conversions, two bits are required in thesignalling message. For eight pad values, three additional bits arerequired. These five bits and the eight bits of the PCM data address thetable 42 and select one of 2¹³ different PCM codes in accordance withthe pad and companding law conversion which is dictated by thesignalling message applied to the conversion control 44 and storedtherein.

The system uses serial to parallel conversion to translate the incomingPCM from serial to parallel data. Another parallel to serial conversionis associated with the table memory 42. The code and pad conversion maytake two clock periods of the PCM clock. Appropriate delays are used inthe signal message transmission paths so as to maintain the properrelationship between the PCM and its associated control signals.Accordingly, the TSI is capable of making the pad and companding lawconversions required to reconcile the transmission characteristics ofthe port circuits which are interconnected through the TSI.

The operation of the system will become more apparent from FIGS. 5A-D.

FIG. 5A depicts a portion of a state transition diagram by means ofwhich the first and second control means could be implemented.

FIGS. 5B, C and D are flow diagrams depicting the processing of threetypical transition subroutines shown in the state diagram, specificallySUBR "A", SUBR "B", and SUBR "C".

Inputs to this processing consist of the "Port Events", some of whichare given in FIG. 5A (column headers), and the current port state (rowheaders) which is read out of a processor memory area for the particularport being processed. The "Port Events" consist of two general types,although the processing for both types is the same. The differencebetween the two types is their origin. One type of "Port Event" arisesfrom external stimuli such as hardware seizure indications or otherindications of the external status of the trunk circuit. The other typeof "Port Event" is actually an "IPM" (InterProcess Message) resultingfrom the execution of a transition subroutine invoked by processing ofthis port or some other port in the system which has an association withthis port.

At the intersection of an event (column) with a state (row) in the statetransition table stored in processor memory (FIG. 5A) is the address ofthe transition subroutine responsible for processing this particularevent/state combination. The subroutine is then executed. Its executiontypically results in a new state being stored in processor memory alongwith one or more output events. Again, some of these outputs aredirected to trunk circuits or the TSI to cause certain hardware actionsto be performed, and other outputs become inputs (IPMs) to the same orother ports such as a trunk associated in a call connection, as depictedin FIGS. 5B, C and D.

Certain of the state transition table entries in FIG. 5A and theirassociated state transition subroutines (similar to those shown in FIGS.5B, C and D) comprise the first control means which is connected withsignalling protocols specific to the trunk type, while the others are ofa more general nature and comprise the second control means, in whichinteractions use a common universal language independent of signallingprotocol.

By convention, the subroutines comprising the first control means willcontain hardware specific processing and are invoked by the protocolspecific events in the table, while the subroutines comprising thesecond control means communicate to their peers only through the use ofa set of IPMs common to that set of subroutines, those IPMs not beingused by the first control means. In this way the latter set of IPMscomprise a universal language, and the first control means is therebyrelieved of the requirement to possess knowledge of the details ofsignalling protocols of all the various trunk types to which a giventrunk may be connected. Adding a new signalling protocol is thus greatlysimplified; in that a state process is used for the protocol using theconventions of the first control means (specific to that protocol) andinterfacing to the peer processes of this and other trunk types usingconventions of the second control means involving IPMs of the universallanguage.

From the foregoing description it will be apparent that there has beenprovided an improved time division multiplex system which provides agateway for interconnecting circuits, such as trunks, originating intelecommunication networks having different signalling, transmission andinsertion loss protocols and formats. Variations and modifications inthe herein described system, within the scope of the invention, willundoubtedly suggest themselves to those skilled in the art. Accordinglythe foregoing description should be taken as illustrative and not in alimiting sense.

We claim:
 1. A time division switching system for providingcommunications between circuits which operate with signalling datasignals formatted according to any of the plurality of differentsignalling protocols and which circuits are connected to highways overwhich message data is transmitted, said system comprising time slotinterchange means connected to said highways for providing time divisionmultiplexed connections for said message data, first control meansresponsive to said signalling data signals for controlling said circuitsin accordance with the signalling protocol in which said data is handledby said circuits, and second control means also response to saidsignalling data signals for translating said signalling datairrespective of the signalling protocol thereof being any of saidplurality of different signalling protocols into signalling messages inaccordance with a protocol common to said circuits for establishing withsaid common protocol message said time division multiplex connections insaid time slot interchange means and for carrying signalling databetween said first control means, and wherein said data messages can betransmitted and received by said circuits in different PCM codesrepresenting said messages in accordance with different companding lawsor attenuation characteristics, said time slot interchange means havingcode conversion means for converting messages switched between saidcircuits into the PCM code for the respective ones of said circuits, andmeans for controlling said conversion means with said messages from saidtranslating means for providing code conversions between codes for saidcircuits between which said data messages are switched by said time slotinterchange means.
 2. A time division switching system for providingcommunications between circuits which operate with signalling datasignals formatted according to any of the plurality of differentsignalling protocols and which circuits are connected to highways overwhich message data is transmitted, said system comprising time slotinterchange means connected to said highways for providing time divisionmultiplexed connections for said message data, first control meansresponsive to said signalling data signals for controlling said circuitsin accordance with the signalling protocol in which said data is handledby said circuits, and second control means also response to saidsignalling data signals for translating said signalling datairrespective of the signalling protocol thereof being any of saidplurality of different signalling protocols into signalling messages inaccordance with a protocol common to said circuits for establishing withsaid common protocol message said time division multiplex connections insaid time slot interchange means and for carrying signalling databetween said first control means, and wherein said first control meansincludes means responsive to the type of said circuit for identifyingsaid circuit which is to be connected through said time slot interchangemeans, said first control means including means for first processing ofsaid signalling data signals in accordance with different protocolscorresponding to a plurality of different types of said circuits, meansfor invoking said processing means which processes said signalling datain accordance with the type of said circuit which is identified by saidtype responsive means, said translating means including means for secondprocessing of said signalling data in response to the type of circuitidentified by said type responsive means to produce said signallingmessages in said common protocol.
 3. The system according to claim 2,wherein said first and second processing means are provided by acomputer having a data processor and memory, said memory having storedtherein a data base containing a plurality of routines, and means foraccessing said routines in accordance with said signalling data signalsand with the type identification of said circuits having said messagedata connections therebetween.
 4. The system according to claim 1wherein said time slot interchange means comprises means for switchingsaid data message between said incoming and outgoing ones of saidhighways, in time slots represented by said signalling messages, andsaid conversion means is connected along said outgoing highway for saiddata messages which are switched to said outgoing highway, and saidconversion means having inputs for said data messages and for saidsignalling messages and outputs for said data messages.
 5. The systemaccording to claim 4 wherein said converting means comprises meansresponsive to said signalling messages for selecting different outputdata messages corresponding to said data messages applied to said inputsthereof.
 6. The system according to claim 5 wherein said conversionmeans includes means providing a look up table containing said outputdata messages, and said selecting means including means responsive tosaid data messages and said signalling messages applied to the inputs ofsaid conversion means for addressing said look up table.